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Re: [PATCH v2 5/6] Add use of RCU for qemu_logfile.
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From: |
Robert Foley |
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Subject: |
Re: [PATCH v2 5/6] Add use of RCU for qemu_logfile. |
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Date: |
Mon, 18 Nov 2019 09:21:41 -0500 |
On Mon, 18 Nov 2019 at 08:23, Alex Bennée <address@hidden> wrote:
>
>
> Robert Foley <address@hidden> writes:
>
> > On Mon, 18 Nov 2019 at 07:22, Alex Bennée <address@hidden> wrote:
> >>
> >>
> >> > + if (logfile) {
> >> > + qemu_flockfile(logfile->fd);
> >> > + return logfile->fd;
> >> > + } else {
> >> > + rcu_read_unlock();
> >>
> >> As qemu_log_lock() and unlock should be paired we can drop the unlock
> >> here and do an unconditional unlock bellow even if a null fd is passed.
> >>
> >> Otherwise:
> >>
> >> Reviewed-by: Alex Bennée <address@hidden>
> >>
> > Sounds reasonable. It sounds like we should change it to be something
> > like this.
> >
> > static inline FILE *qemu_log_lock(void)
> > {
> > QemuLogFile *logfile;
> > rcu_read_lock();
> > logfile = atomic_rcu_read(&qemu_logfile);
> > if (logfile) {
> > qemu_flockfile(logfile->fd);
> > return logfile->fd;
> > }
> > rcu_read_unlock();
> > return NULL;
> > }
> > I will make the changes.
>
> No I mean as you have to call qemu_log_unlock() you can to the unlock
> there. You have to hold the lock over the usage of the resource:
>
OK, got it.
We will need to make one other change to target/tilegx/translate.c
since it conditionally unlocks depending on what qemu_log_lock()
returns.
It seems like the right thing to do since it makes all uses of
qemu_log_lock() homogeneous in that they will always unconditionally
call qemu_log_unlock().
I will make the changes.
Thanks,
-Rob
On Mon, 18 Nov 2019 at 08:23, Alex Bennée <address@hidden> wrote:
>
>
> Robert Foley <address@hidden> writes:
>
> > On Mon, 18 Nov 2019 at 07:22, Alex Bennée <address@hidden> wrote:
> >>
> >>
> >> > + if (logfile) {
> >> > + qemu_flockfile(logfile->fd);
> >> > + return logfile->fd;
> >> > + } else {
> >> > + rcu_read_unlock();
> >>
> >> As qemu_log_lock() and unlock should be paired we can drop the unlock
> >> here and do an unconditional unlock bellow even if a null fd is passed.
> >>
> >> Otherwise:
> >>
> >> Reviewed-by: Alex Bennée <address@hidden>
> >>
> > Sounds reasonable. It sounds like we should change it to be something
> > like this.
> >
> > static inline FILE *qemu_log_lock(void)
> > {
> > QemuLogFile *logfile;
> > rcu_read_lock();
> > logfile = atomic_rcu_read(&qemu_logfile);
> > if (logfile) {
> > qemu_flockfile(logfile->fd);
> > return logfile->fd;
> > }
> > rcu_read_unlock();
> > return NULL;
> > }
> > I will make the changes.
>
> No I mean as you have to call qemu_log_unlock() you can to the unlock
> there. You have to hold the lock over the usage of the resource:
>
> static inline FILE *qemu_log_lock(void)
> {
> QemuLogFile *logfile;
> rcu_read_lock();
> logfile = atomic_rcu_read(&qemu_logfile);
> if (logfile) {
> qemu_flockfile(logfile->fd);
> return logfile->fd;
> } else {
> return NULL;
> }
> }
>
> static inline void qemu_log_unlock(FILE *fd)
> {
> if (fd) {
> qemu_funlockfile(fd);
> }
> rcu_read_unlock();
> }
>
>
> >
> > Thanks,
> > -Rob
> > On Mon, 18 Nov 2019 at 07:22, Alex Bennée <address@hidden> wrote:
> >>
> >>
> >> Robert Foley <address@hidden> writes:
> >>
> >> > This now allows changing the logfile while logging is active,
> >> > and also solves the issue of a seg fault while changing the logfile.
> >> >
> >> > Any read access to the qemu_logfile handle will use
> >> > the rcu_read_lock()/unlock() around the use of the handle.
> >> > To fetch the handle we will use atomic_rcu_read().
> >> > We also in many cases do a check for validity of the
> >> > logfile handle before using it to deal with the case where the
> >> > file is closed and set to NULL.
> >> >
> >> > The cases where we write to the qemu_logfile will use atomic_rcu_set().
> >> > Writers will also use call_rcu() with a newly added qemu_logfile_free
> >> > function for freeing/closing when readers have finished.
> >> >
> >> > Signed-off-by: Robert Foley <address@hidden>
> >> > ---
> >> > v2
> >> > - No specific changes, just merging in cleanup changes in
> >> > qemu_set_log().
> >> > ---
> >> > v1
> >> > - Changes for review comments.
> >> > - Minor changes to definition of QemuLogFile.
> >> > - Changed qemu_log_separate() to fix unbalanced and
> >> > remove qemu_log_enabled() check.
> >> > - changed qemu_log_lock() to include else.
> >> > - make qemu_logfile_free static.
> >> > - use g_assert(logfile) in qemu_logfile_free.
> >> > - Relocated unlock out of if/else in qemu_log_close(), and
> >> > in qemu_set_log().
> >> > ---
> >> > include/qemu/log.h | 42 +++++++++++++++++++++++----
> >> > util/log.c | 72 ++++++++++++++++++++++++++++++++--------------
> >> > include/exec/log.h | 33 ++++++++++++++++++---
> >> > tcg/tcg.c | 12 ++++++--
> >> > 4 files changed, 126 insertions(+), 33 deletions(-)
> >> >
> >> > diff --git a/include/qemu/log.h b/include/qemu/log.h
> >> > index a7c5b01571..528e1f9dd7 100644
> >> > --- a/include/qemu/log.h
> >> > +++ b/include/qemu/log.h
> >> > @@ -3,9 +3,16 @@
> >> >
> >> > /* A small part of this API is split into its own header */
> >> > #include "qemu/log-for-trace.h"
> >> > +#include "qemu/rcu.h"
> >> > +
> >> > +typedef struct QemuLogFile {
> >> > + struct rcu_head rcu;
> >> > + FILE *fd;
> >> > +} QemuLogFile;
> >> >
> >> > /* Private global variable, don't use */
> >> > -extern FILE *qemu_logfile;
> >> > +extern QemuLogFile *qemu_logfile;
> >> > +
> >> >
> >> > /*
> >> > * The new API:
> >> > @@ -25,7 +32,16 @@ static inline bool qemu_log_enabled(void)
> >> > */
> >> > static inline bool qemu_log_separate(void)
> >> > {
> >> > - return qemu_logfile != NULL && qemu_logfile != stderr;
> >> > + QemuLogFile *logfile;
> >> > + bool res = false;
> >> > +
> >> > + rcu_read_lock();
> >> > + logfile = atomic_rcu_read(&qemu_logfile);
> >> > + if (logfile && logfile->fd != stderr) {
> >> > + res = true;
> >> > + }
> >> > + rcu_read_unlock();
> >> > + return res;
> >> > }
> >> >
> >> > #define CPU_LOG_TB_OUT_ASM (1 << 0)
> >> > @@ -55,14 +71,23 @@ static inline bool qemu_log_separate(void)
> >> >
> >> > static inline FILE *qemu_log_lock(void)
> >> > {
> >> > - qemu_flockfile(qemu_logfile);
> >> > - return logfile->fd;
> >> > + QemuLogFile *logfile;
> >> > + rcu_read_lock();
> >> > + logfile = atomic_rcu_read(&qemu_logfile);
> >> > + if (logfile) {
> >> > + qemu_flockfile(logfile->fd);
> >> > + return logfile->fd;
> >> > + } else {
> >> > + rcu_read_unlock();
> >>
> >> As qemu_log_lock() and unlock should be paired we can drop the unlock
> >> here and do an unconditional unlock bellow even if a null fd is passed.
> >>
> >> Otherwise:
> >>
> >> Reviewed-by: Alex Bennée <address@hidden>
> >>
> >> > + return NULL;
> >> > + }
> >> > }
> >> >
> >> > static inline void qemu_log_unlock(FILE *fd)
> >> > {
> >> > if (fd) {
> >> > qemu_funlockfile(fd);
> >> > + rcu_read_unlock();
> >> > }
> >> > }
> >> >
> >> > @@ -73,9 +98,14 @@ static inline void qemu_log_unlock(FILE *fd)
> >> > static inline void GCC_FMT_ATTR(1, 0)
> >> > qemu_log_vprintf(const char *fmt, va_list va)
> >> > {
> >> > - if (qemu_logfile) {
> >> > - vfprintf(qemu_logfile, fmt, va);
> >> > + QemuLogFile *logfile;
> >> > +
> >> > + rcu_read_lock();
> >> > + logfile = atomic_rcu_read(&qemu_logfile);
> >> > + if (logfile) {
> >> > + vfprintf(logfile->fd, fmt, va);
> >> > }
> >> > + rcu_read_unlock();
> >> > }
> >> >
> >> > /* log only if a bit is set on the current loglevel mask:
> >> > diff --git a/util/log.c b/util/log.c
> >> > index 91ebb5c924..9f9b6b74b7 100644
> >> > --- a/util/log.c
> >> > +++ b/util/log.c
> >> > @@ -28,7 +28,7 @@
> >> >
> >> > static char *logfilename;
> >> > static QemuMutex qemu_logfile_mutex;
> >> > -FILE *qemu_logfile;
> >> > +QemuLogFile *qemu_logfile;
> >> > int qemu_loglevel;
> >> > static int log_append = 0;
> >> > static GArray *debug_regions;
> >> > @@ -37,10 +37,14 @@ static GArray *debug_regions;
> >> > int qemu_log(const char *fmt, ...)
> >> > {
> >> > int ret = 0;
> >> > - if (qemu_logfile) {
> >> > + QemuLogFile *logfile;
> >> > +
> >> > + rcu_read_lock();
> >> > + logfile = atomic_rcu_read(&qemu_logfile);
> >> > + if (logfile) {
> >> > va_list ap;
> >> > va_start(ap, fmt);
> >> > - ret = vfprintf(qemu_logfile, fmt, ap);
> >> > + ret = vfprintf(logfile->fd, fmt, ap);
> >> > va_end(ap);
> >> >
> >> > /* Don't pass back error results. */
> >> > @@ -48,6 +52,7 @@ int qemu_log(const char *fmt, ...)
> >> > ret = 0;
> >> > }
> >> > }
> >> > + rcu_read_unlock();
> >> > return ret;
> >> > }
> >> >
> >> > @@ -56,12 +61,24 @@ static void __attribute__((__constructor__))
> >> > qemu_logfile_init(void)
> >> > qemu_mutex_init(&qemu_logfile_mutex);
> >> > }
> >> >
> >> > +static void qemu_logfile_free(QemuLogFile *logfile)
> >> > +{
> >> > + g_assert(logfile);
> >> > +
> >> > + if (logfile->fd != stderr) {
> >> > + fclose(logfile->fd);
> >> > + }
> >> > + g_free(logfile);
> >> > +}
> >> > +
> >> > static bool log_uses_own_buffers;
> >> >
> >> > /* enable or disable low levels log */
> >> > void qemu_set_log(int log_flags)
> >> > {
> >> > bool need_to_open_file = false;
> >> > + QemuLogFile *logfile;
> >> > +
> >> > qemu_loglevel = log_flags;
> >> > #ifdef CONFIG_TRACE_LOG
> >> > qemu_loglevel |= LOG_TRACE;
> >> > @@ -80,43 +97,47 @@ void qemu_set_log(int log_flags)
> >> > g_assert(qemu_logfile_mutex.initialized);
> >> > qemu_mutex_lock(&qemu_logfile_mutex);
> >> > if (qemu_logfile && !need_to_open_file) {
> >> > - qemu_mutex_unlock(&qemu_logfile_mutex);
> >> > - qemu_log_close();
> >> > + logfile = qemu_logfile;
> >> > + atomic_rcu_set(&qemu_logfile, NULL);
> >> > + call_rcu(logfile, qemu_logfile_free, rcu);
> >> > } else if (!qemu_logfile && need_to_open_file) {
> >> > + logfile = g_new0(QemuLogFile, 1);
> >> > if (logfilename) {
> >> > - qemu_logfile = fopen(logfilename, log_append ? "a" : "w");
> >> > - if (!qemu_logfile) {
> >> > + logfile->fd = fopen(logfilename, log_append ? "a" : "w");
> >> > + if (!logfile->fd) {
> >> > + g_free(logfile);
> >> > perror(logfilename);
> >> > _exit(1);
> >> > }
> >> > /* In case we are a daemon redirect stderr to logfile */
> >> > if (is_daemonized()) {
> >> > - dup2(fileno(qemu_logfile), STDERR_FILENO);
> >> > - fclose(qemu_logfile);
> >> > + dup2(fileno(logfile->fd), STDERR_FILENO);
> >> > + fclose(logfile->fd);
> >> > /* This will skip closing logfile in qemu_log_close() */
> >> > - qemu_logfile = stderr;
> >> > + logfile->fd = stderr;
> >> > }
> >> > } else {
> >> > /* Default to stderr if no log file specified */
> >> > assert(!is_daemonized());
> >> > - qemu_logfile = stderr;
> >> > + logfile->fd = stderr;
> >> > }
> >> > /* must avoid mmap() usage of glibc by setting a buffer "by
> >> > hand" */
> >> > if (log_uses_own_buffers) {
> >> > static char logfile_buf[4096];
> >> >
> >> > - setvbuf(qemu_logfile, logfile_buf, _IOLBF,
> >> > sizeof(logfile_buf));
> >> > + setvbuf(logfile->fd, logfile_buf, _IOLBF,
> >> > sizeof(logfile_buf));
> >> > } else {
> >> > #if defined(_WIN32)
> >> > /* Win32 doesn't support line-buffering, so use unbuffered
> >> > output. */
> >> > - setvbuf(qemu_logfile, NULL, _IONBF, 0);
> >> > + setvbuf(logfile->fd, NULL, _IONBF, 0);
> >> > #else
> >> > - setvbuf(qemu_logfile, NULL, _IOLBF, 0);
> >> > + setvbuf(logfile->fd, NULL, _IOLBF, 0);
> >> > #endif
> >> > log_append = 1;
> >> > }
> >> > - qemu_mutex_unlock(&qemu_logfile_mutex);
> >> > + atomic_rcu_set(&qemu_logfile, logfile);
> >> > }
> >> > + qemu_mutex_unlock(&qemu_logfile_mutex);
> >> > }
> >> >
> >> > void qemu_log_needs_buffers(void)
> >> > @@ -245,19 +266,28 @@ out:
> >> > /* fflush() the log file */
> >> > void qemu_log_flush(void)
> >> > {
> >> > - fflush(qemu_logfile);
> >> > + QemuLogFile *logfile;
> >> > +
> >> > + rcu_read_lock();
> >> > + logfile = atomic_rcu_read(&qemu_logfile);
> >> > + if (logfile) {
> >> > + fflush(logfile->fd);
> >> > + }
> >> > + rcu_read_unlock();
> >> > }
> >> >
> >> > /* Close the log file */
> >> > void qemu_log_close(void)
> >> > {
> >> > + QemuLogFile *logfile;
> >> > +
> >> > g_assert(qemu_logfile_mutex.initialized);
> >> > qemu_mutex_lock(&qemu_logfile_mutex);
> >> > - if (qemu_logfile) {
> >> > - if (qemu_logfile != stderr) {
> >> > - fclose(qemu_logfile);
> >> > - }
> >> > - qemu_logfile = NULL;
> >> > + logfile = qemu_logfile;
> >> > +
> >> > + if (logfile) {
> >> > + atomic_rcu_set(&qemu_logfile, NULL);
> >> > + call_rcu(logfile, qemu_logfile_free, rcu);
> >> > }
> >> > qemu_mutex_unlock(&qemu_logfile_mutex);
> >> > }
> >> > diff --git a/include/exec/log.h b/include/exec/log.h
> >> > index e2cfd436e6..9bd1e4aa20 100644
> >> > --- a/include/exec/log.h
> >> > +++ b/include/exec/log.h
> >> > @@ -15,8 +15,15 @@
> >> > */
> >> > static inline void log_cpu_state(CPUState *cpu, int flags)
> >> > {
> >> > + QemuLogFile *logfile;
> >> > +
> >> > if (qemu_log_enabled()) {
> >> > - cpu_dump_state(cpu, qemu_logfile, flags);
> >> > + rcu_read_lock();
> >> > + logfile = atomic_rcu_read(&qemu_logfile);
> >> > + if (logfile) {
> >> > + cpu_dump_state(cpu, logfile->fd, flags);
> >> > + }
> >> > + rcu_read_unlock();
> >> > }
> >> > }
> >> >
> >> > @@ -40,19 +47,37 @@ static inline void log_cpu_state_mask(int mask,
> >> > CPUState *cpu, int flags)
> >> > static inline void log_target_disas(CPUState *cpu, target_ulong start,
> >> > target_ulong len)
> >> > {
> >> > - target_disas(qemu_logfile, cpu, start, len);
> >> > + QemuLogFile *logfile;
> >> > + rcu_read_lock();
> >> > + logfile = atomic_rcu_read(&qemu_logfile);
> >> > + if (logfile) {
> >> > + target_disas(logfile->fd, cpu, start, len);
> >> > + }
> >> > + rcu_read_unlock();
> >> > }
> >> >
> >> > static inline void log_disas(void *code, unsigned long size)
> >> > {
> >> > - disas(qemu_logfile, code, size);
> >> > + QemuLogFile *logfile;
> >> > + rcu_read_lock();
> >> > + logfile = atomic_rcu_read(&qemu_logfile);
> >> > + if (logfile) {
> >> > + disas(logfile->fd, code, size);
> >> > + }
> >> > + rcu_read_unlock();
> >> > }
> >> >
> >> > #if defined(CONFIG_USER_ONLY)
> >> > /* page_dump() output to the log file: */
> >> > static inline void log_page_dump(void)
> >> > {
> >> > - page_dump(qemu_logfile);
> >> > + QemuLogFile *logfile;
> >> > + rcu_read_lock();
> >> > + logfile = atomic_rcu_read(&qemu_logfile);
> >> > + if (logfile) {
> >> > + page_dump(logfile->fd);
> >> > + }
> >> > + rcu_read_unlock();
> >> > }
> >> > #endif
> >> > #endif
> >> > diff --git a/tcg/tcg.c b/tcg/tcg.c
> >> > index 0511266d85..4f616ba38b 100644
> >> > --- a/tcg/tcg.c
> >> > +++ b/tcg/tcg.c
> >> > @@ -2114,9 +2114,17 @@ static void tcg_dump_ops(TCGContext *s, bool
> >> > have_prefs)
> >> > }
> >> >
> >> > if (have_prefs || op->life) {
> >> > - for (; col < 40; ++col) {
> >> > - putc(' ', qemu_logfile);
> >> > +
> >> > + QemuLogFile *logfile;
> >> > +
> >> > + rcu_read_lock();
> >> > + logfile = atomic_rcu_read(&qemu_logfile);
> >> > + if (logfile) {
> >> > + for (; col < 40; ++col) {
> >> > + putc(' ', logfile->fd);
> >> > + }
> >> > }
> >> > + rcu_read_unlock();
> >> > }
> >> >
> >> > if (op->life) {
> >>
> >>
> >> --
> >> Alex Bennée
>
>
> --
> Alex Bennée
- [PATCH v2 3/6] Add a mutex to guarantee single writer to qemu_logfile handle., (continued)
[PATCH v2 6/6] Added tests for close and change of logfile., Robert Foley, 2019/11/15