Hi Wei Fang,
On Wed, Jun 22, 2016 at 11:01:15AM +0800, Wei Fang wrote:
> We triggered soft-lockup under stress test which
> open/access/write/close one file concurrently on more than
> five different CPUs:
>
> WARN: soft lockup - CPU#0 stuck for 11s! [who:30631]
> ...
> [<ffffffc0003986f8>] dput+0x100/0x298
> [<ffffffc00038c2dc>] terminate_walk+0x4c/0x60
> [<ffffffc00038f56c>] path_lookupat+0x5cc/0x7a8
> [<ffffffc00038f780>] filename_lookup+0x38/0xf0
> [<ffffffc000391180>] user_path_at_empty+0x78/0xd0
> [<ffffffc0003911f4>] user_path_at+0x1c/0x28
> [<ffffffc00037d4fc>] SyS_faccessat+0xb4/0x230
>
> ->d_lock trylock may failed many times because of concurrently
> operations, and dput() may execute a long time.
>
> Fix this by replacing cpu_relax() with cond_resched().
> dput() used to be sleepable, so make it sleepable again
> should be safe.
>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Wei Fang <fangwei1@xxxxxxxxxx>
> ---
> Changes v1->v2:
> - add might_sleep() to annotate that dput() can sleep
>
> fs/dcache.c | 4 +++-
> 1 files changed, 3 insertions(+), 1 deletions(-)
>
> diff --git a/fs/dcache.c b/fs/dcache.c
> index d5ecc6e..074fc1c 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -578,7 +578,7 @@ static struct dentry *dentry_kill(struct dentry *dentry)
>
> failed:
> spin_unlock(&dentry->d_lock);
> - cpu_relax();
> + cond_resched();
Is it better to put the cond_resched() in the caller(i.e. dput()), right
before "goto repeat"? Because it's obviously a loop there, which makes
the purpose of cond_resched() more straightforward.
Regards,
Boqun
> return dentry; /* try again with same dentry */
> }
>
> @@ -752,6 +752,8 @@ void dput(struct dentry *dentry)
> return;
>
> repeat:
> + might_sleep();
> +
> rcu_read_lock();
> if (likely(fast_dput(dentry))) {
> rcu_read_unlock();
> --
> 1.7.1
>
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