* [PATCH] fs: fix a hungtask problem when freeze/unfreeze fs
@ 2021-01-05 13:46 Jan Kara
2021-01-21 2:00 ` Shijie Luo
0 siblings, 1 reply; 2+ messages in thread
From: Jan Kara @ 2021-01-05 13:46 UTC (permalink / raw)
To: Al Viro; +Cc: linux-fsdevel, Shijie Luo, Jan Kara
We found the following deadlock when running xfstests generic/390 with ext4
filesystem, and simutaneously offlining/onlining the disk we tested. It will
cause a deadlock whose call trace is like this:
fsstress D 0 11672 11625 0x00000080
Call Trace:
? __schedule+0x2fc/0x930
? filename_parentat+0x10b/0x1a0
schedule+0x28/0x70
rwsem_down_read_failed+0x102/0x1c0
? __percpu_down_read+0x93/0xb0
__percpu_down_read+0x93/0xb0
__sb_start_write+0x5f/0x70
mnt_want_write+0x20/0x50
do_renameat2+0x1f3/0x550
__x64_sys_rename+0x1c/0x20
do_syscall_64+0x5b/0x1b0
entry_SYSCALL_64_after_hwframe+0x65/0xca
The root cause is that when ext4 hits IO error due to disk being
offline, it will switch itself into read-only state. When it is frozen
at that moment, following thaw_super() call will not unlock percpu
freeze semaphores (as the fs is read-only) causing the deadlock.
Fix the problem by tracking whether the superblock was read-only at the
time we were freezing it.
Reported-and-tested-by: Shijie Luo <luoshijie1@huawei.com>
Signed-off-by: Jan Kara <jack@suse.cz>
---
fs/super.c | 9 ++++++++-
include/linux/fs.h | 4 +++-
2 files changed, 11 insertions(+), 2 deletions(-)
Al, can you pick up this patch please? Thanks!
diff --git a/fs/super.c b/fs/super.c
index 2c6cdea2ab2d..c35a2938ee99 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1674,10 +1674,12 @@ int freeze_super(struct super_block *sb)
if (sb_rdonly(sb)) {
/* Nothing to do really... */
sb->s_writers.frozen = SB_FREEZE_COMPLETE;
+ sb->s_writers.frozen_ro = 1;
up_write(&sb->s_umount);
return 0;
}
+ sb->s_writers.frozen_ro = 0;
sb->s_writers.frozen = SB_FREEZE_WRITE;
/* Release s_umount to preserve sb_start_write -> s_umount ordering */
up_write(&sb->s_umount);
@@ -1733,7 +1735,12 @@ static int thaw_super_locked(struct super_block *sb)
return -EINVAL;
}
- if (sb_rdonly(sb)) {
+ /*
+ * Was the fs frozen in read-only state? Note that we cannot just check
+ * sb_rdonly(sb) as the filesystem might have switched to read-only
+ * state due to internal errors or so.
+ */
+ if (sb->s_writers.frozen_ro) {
sb->s_writers.frozen = SB_UNFROZEN;
goto out;
}
diff --git a/include/linux/fs.h b/include/linux/fs.h
index ad4cf1bae586..346ab981128f 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1406,7 +1406,9 @@ enum {
#define SB_FREEZE_LEVELS (SB_FREEZE_COMPLETE - 1)
struct sb_writers {
- int frozen; /* Is sb frozen? */
+ unsigned short frozen; /* Is sb frozen? */
+ unsigned short frozen_ro; /* Was sb read-only
+ * when frozen? */
wait_queue_head_t wait_unfrozen; /* wait for thaw */
struct percpu_rw_semaphore rw_sem[SB_FREEZE_LEVELS];
};
--
2.26.2
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH] fs: fix a hungtask problem when freeze/unfreeze fs
2021-01-05 13:46 [PATCH] fs: fix a hungtask problem when freeze/unfreeze fs Jan Kara
@ 2021-01-21 2:00 ` Shijie Luo
0 siblings, 0 replies; 2+ messages in thread
From: Shijie Luo @ 2021-01-21 2:00 UTC (permalink / raw)
To: Jan Kara, Al Viro; +Cc: linux-fsdevel
friendly ping...
On 2021/1/5 21:46, Jan Kara wrote:
> We found the following deadlock when running xfstests generic/390 with ext4
> filesystem, and simutaneously offlining/onlining the disk we tested. It will
> cause a deadlock whose call trace is like this:
>
> fsstress D 0 11672 11625 0x00000080
> Call Trace:
> ? __schedule+0x2fc/0x930
> ? filename_parentat+0x10b/0x1a0
> schedule+0x28/0x70
> rwsem_down_read_failed+0x102/0x1c0
> ? __percpu_down_read+0x93/0xb0
> __percpu_down_read+0x93/0xb0
> __sb_start_write+0x5f/0x70
> mnt_want_write+0x20/0x50
> do_renameat2+0x1f3/0x550
> __x64_sys_rename+0x1c/0x20
> do_syscall_64+0x5b/0x1b0
> entry_SYSCALL_64_after_hwframe+0x65/0xca
>
> The root cause is that when ext4 hits IO error due to disk being
> offline, it will switch itself into read-only state. When it is frozen
> at that moment, following thaw_super() call will not unlock percpu
> freeze semaphores (as the fs is read-only) causing the deadlock.
>
>
> };
^ permalink raw reply [flat|nested] 2+ messages in thread
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