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Re: deadlock with &log->l_cilp->xc_ctx_lock semaphone

To: Chandra Seetharaman <sekharan@xxxxxxxxxx>
Subject: Re: deadlock with &log->l_cilp->xc_ctx_lock semaphone
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Thu, 23 May 2013 09:41:29 +1000
Cc: XFS mailing list <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1369264363.10223.2994.camel@xxxxxxxxxxxxxxxxxx>
References: <1369264363.10223.2994.camel@xxxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Wed, May 22, 2013 at 06:12:43PM -0500, Chandra Seetharaman wrote:
> Hello,
> 
> While testing and rearranging my pquota/gquota code, I stumbled on a
> xfs_shutdown() during a mount. But the mount just hung.
> 
> I debugged and found that it is in a code path where
> &log->l_cilp->xc_ctx_lock is first acquired in read mode and some levels
> down the same semaphore is being acquired in write mode causing a
> deadlock.
> 
> This is the stack:
> xfs_log_commit_cil -> acquires &log->l_cilp->xc_ctx_lock in read mode
>   xlog_print_tic_res
>     xfs_force_shutdown
>       xfs_log_force_umount
>         xlog_cil_force
>           xlog_cil_force_lsn
>             xlog_cil_push_foreground
>               xlog_cil_push - tries to acquire same semaphore in write mode

Which means you had a transaction reservation overrun. Is it
reproducable? iDo you have the output from xlog_print_tic_res()?
Because:

> xfs_trans_commit+0x79/0x270 [xfs]  
> xfs_qm_write_sb_changes+0x61/0x90 [xfs]
> xfs_qm_mount_quotas+0x82/0x180 [xfs]
> xfs_mountfs+0x5f6/0x6b0 [xfs]

This transaction only modifies the superblock, and it has a buffer
reservation for a superblock sized buffer, and hence should never
overrun.

IOWs, I'm ifar more concerned about the fact there was a
transaction overrun than they was a hang in the path that handles
the overrun. The fact this hang has been there since 2.6.35 tells
you how rare transactions overruns are....

FWIW, the fix for the hang is to make xlog_print_tic_res() return an
error and have the caller handle the shutdown.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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