RE: [PATCH] vfs: Fix RCU path walk failiures due to uninitializednameidata seq number for root directory

[Shi, Alex]

This can fix dbenchthreads/aim7 regressions from 39-rc1 kernel, that caused by path_init_rcu/path_init merge. 


Regards! 
Alex  
>-----Original Message-----
>From: Tim Chen [mailto:tim.c.chen@xxxxxxxxxxxxxxx]
>Sent: Saturday, April 16, 2011 2:39 AM
>To: Alexander Viro; Nick Piggin
>Cc: Andi Kleen; linux-fsdevel@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Li, Shaohua; Shi, Alex
>Subject: [PATCH] vfs: Fix RCU path walk failiures due to uninitialized nameidata seq number for root directory
>
>During RCU walk in path_lookupat and path_openat, the rcu lookup
>frequently failed because when root directory was looked up, seq number
>was not properly set in nameidata.  We dropped out of RCU walk in
>nameidata_drop_rcu due to mismatch in directory entry's seq number.  We
>reverted to slow path walk that need to take references.
>
>With the following patch, I saw a 50% increase in an exim mail server
>benchmark throughput on a 4-socket Nehalem-EX system.
>
>Thanks.
>
>Tim
>
>Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
>diff --git a/fs/namei.c b/fs/namei.c
>index 3cb616d..e4b27a6 100644
>--- a/fs/namei.c
>+++ b/fs/namei.c
>@@ -697,6 +697,7 @@ static __always_inline void set_root_rcu(struct nameidata *nd)
> 		do {
> 			seq = read_seqcount_begin(&fs->seq);
> 			nd->root = fs->root;
>+			nd->seq = __read_seqcount_begin(&nd->root.dentry->d_seq);
> 		} while (read_seqcount_retry(&fs->seq, seq));
> 	}
> }
>

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