[Qemu-devel] [PATCH 3/7] pstore: Manage buffer position for async write

[Namhyung Kim]

Some pstore backend only supports async mode, so it's possible to do
some IO at the same time.  In this case we need to manage the single
psinfo->buf not to broken by concurrent IO.

For example PSTORE_TYPE_CONSOLE IO is serialized by psinfo->buf_lock but
later IO can be issued before finishing previous request.  In this case
it overwrites psinfo->buf so the previous result might be broken.

This patch adds psinfo->bufpos field to keep track of current position
in order to use psinfo->buf as a ring buffer.  However it's just a
simple, best-effort way of doing that, and provides no 100% guarantee.
It's only effective for small concurrent IO like PSTORE_TYPE_CONSOLE
IMHO.

The new PSTORE_FLAGS_ASYNC flag enables management of buffer position.
The pstore_prepare_buf() is called before accessing the psinfo->buf and
the pstore_update_buf() is called after accessing the buf.

The pstore_get_buf() is provided for psinfo->write callback to determine
the current position of available buffer.

Cc: Anton Vorontsov <address@hidden>
Cc: Colin Cross <address@hidden>
Cc: Kees Cook <address@hidden>
Cc: Tony Luck <address@hidden>
Cc: Matt Fleming <address@hidden>
Cc: address@hidden
Signed-off-by: Namhyung Kim <address@hidden>
---
 drivers/firmware/efi/efi-pstore.c |  2 +-
 fs/pstore/platform.c              | 48 +++++++++++++++++++++++++++++++--------
 include/linux/pstore.h            |  4 ++++
 3 files changed, 44 insertions(+), 10 deletions(-)

diff --git a/drivers/firmware/efi/efi-pstore.c 
b/drivers/firmware/efi/efi-pstore.c
index 4daa5acd9117..ae0ffe259e07 100644
--- a/drivers/firmware/efi/efi-pstore.c
+++ b/drivers/firmware/efi/efi-pstore.c
@@ -259,7 +259,7 @@ static int efi_pstore_write(enum pstore_type_id type,
 
        efivar_entry_set_safe(efi_name, vendor, PSTORE_EFI_ATTRIBUTES,
                              !pstore_cannot_block_path(reason),
-                             size, psi->buf);
+                             size, pstore_get_buf(psi));
 
        if (reason == KMSG_DUMP_OOPS)
                efivar_run_worker();
diff --git a/fs/pstore/platform.c b/fs/pstore/platform.c
index 76dd604a0f2c..26e2808cf554 100644
--- a/fs/pstore/platform.c
+++ b/fs/pstore/platform.c
@@ -150,6 +150,27 @@ bool pstore_cannot_block_path(enum kmsg_dump_reason reason)
 }
 EXPORT_SYMBOL_GPL(pstore_cannot_block_path);
 
+static void *pstore_prepare_buf(struct pstore_info *psi, size_t len)
+{
+       if (psi->bufpos + len > psi->bufsize ||
+           (psi->flags & PSTORE_FLAGS_ASYNC) == 0)
+               psi->bufpos = 0;
+
+       return psi->buf + psi->bufpos;
+}
+
+static void pstore_update_buf(struct pstore_info *psi, size_t len)
+{
+       if (psi->flags & PSTORE_FLAGS_ASYNC)
+               psi->bufpos += len;
+}
+
+void *pstore_get_buf(struct pstore_info *psi)
+{
+       return psi->buf + psi->bufpos;
+}
+EXPORT_SYMBOL_GPL(pstore_get_buf);
+
 #ifdef CONFIG_PSTORE_ZLIB_COMPRESS
 /* Derived from logfs_compress() */
 static int compress_zlib(const void *in, void *out, size_t inlen, size_t 
outlen)
@@ -455,18 +476,21 @@ static size_t copy_kmsg_to_buffer(int hsize, size_t len)
 {
        size_t total_len;
        size_t diff;
+       void *dst;
 
        total_len = hsize + len;
+       dst = pstore_prepare_buf(psinfo, total_len);
 
        if (total_len > psinfo->bufsize) {
                diff = total_len - psinfo->bufsize + hsize;
-               memcpy(psinfo->buf, big_oops_buf, hsize);
-               memcpy(psinfo->buf + hsize, big_oops_buf + diff,
+               memcpy(dst, big_oops_buf, hsize);
+               memcpy(dst + hsize, big_oops_buf + diff,
                                        psinfo->bufsize - hsize);
                total_len = psinfo->bufsize;
        } else
-               memcpy(psinfo->buf, big_oops_buf, total_len);
+               memcpy(dst, big_oops_buf, total_len);
 
+       pstore_update_buf(psinfo, total_len);
        return total_len;
 }
 
@@ -500,7 +524,7 @@ static void pstore_dump(struct kmsg_dumper *dumper,
        }
        oopscount++;
        while (total < kmsg_bytes) {
-               char *dst;
+               char *dst, *buf;
                unsigned long size;
                int hsize;
                int zipped_len = -1;
@@ -514,6 +538,7 @@ static void pstore_dump(struct kmsg_dumper *dumper,
                } else {
                        dst = psinfo->buf;
                        size = psinfo->bufsize;
+                       psinfo->bufpos = 0;
                }
 
                hsize = sprintf(dst, "%s#%d Part%u\n", why, oopscount, part);
@@ -524,8 +549,9 @@ static void pstore_dump(struct kmsg_dumper *dumper,
                        break;
 
                if (big_oops_buf && is_locked) {
-                       zipped_len = pstore_compress(dst, psinfo->buf,
-                                               hsize + len, psinfo->bufsize);
+                       buf = pstore_prepare_buf(psinfo, hsize + len);
+                       zipped_len = pstore_compress(dst, buf, hsize + len,
+                                               psinfo->bufsize - 
psinfo->bufpos);
 
                        if (zipped_len > 0) {
                                compressed = true;
@@ -543,6 +569,7 @@ static void pstore_dump(struct kmsg_dumper *dumper,
                        pstore_new_entry = 1;
 
                total += total_len;
+               pstore_update_buf(psinfo, total_len);
                part++;
        }
        if (is_locked)
@@ -573,6 +600,7 @@ static void pstore_console_write(struct console *con, const 
char *s, unsigned c)
 
        while (s < e) {
                unsigned long flags;
+               void *dst;
                u64 id;
 
                if (c > psinfo->bufsize)
@@ -584,8 +612,10 @@ static void pstore_console_write(struct console *con, 
const char *s, unsigned c)
                } else {
                        spin_lock_irqsave(&psinfo->buf_lock, flags);
                }
-               memcpy(psinfo->buf, s, c);
+               dst = pstore_prepare_buf(psinfo, c);
+               memcpy(dst, s, c);
                psinfo->write(PSTORE_TYPE_CONSOLE, 0, &id, 0, 0, 0, c, psinfo);
+               pstore_update_buf(psinfo, c);
                spin_unlock_irqrestore(&psinfo->buf_lock, flags);
                s += c;
                c = e - s;
@@ -619,8 +649,8 @@ static int pstore_write_compat(enum pstore_type_id type,
                               bool compressed, size_t size,
                               struct pstore_info *psi)
 {
-       return psi->write_buf(type, reason, id, part, psinfo->buf, compressed,
-                            size, psi);
+       return psi->write_buf(type, reason, id, part, pstore_get_buf(psinfo),
+                             compressed, size, psi);
 }
 
 /*
diff --git a/include/linux/pstore.h b/include/linux/pstore.h
index 9790904de6d2..14f524177b1f 100644
--- a/include/linux/pstore.h
+++ b/include/linux/pstore.h
@@ -52,6 +52,7 @@ struct pstore_info {
        spinlock_t      buf_lock;       /* serialize access to 'buf' */
        char            *buf;
        size_t          bufsize;
+       size_t          bufpos;
        struct mutex    read_mutex;     /* serialize open/read/close */
        int             flags;
        int             (*open)(struct pstore_info *psi);
@@ -79,8 +80,11 @@ struct pstore_info {
 #define PSTORE_FLAGS_FTRACE    (1 << 2)
 #define PSTORE_FLAGS_PMSG      (1 << 3)
 
+#define PSTORE_FLAGS_ASYNC      (1 << 30)
+
 extern int pstore_register(struct pstore_info *);
 extern void pstore_unregister(struct pstore_info *);
 extern bool pstore_cannot_block_path(enum kmsg_dump_reason reason);
+extern void *pstore_get_buf(struct pstore_info *);
 
 #endif /*_LINUX_PSTORE_H*/
-- 
2.8.0