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Date: Tue, 5 Oct 2021 18:39:59 +0530 From: Shreeya Patel <shreeya.patel@...labora.com> To: Al Viro <viro@...iv.linux.org.uk> Cc: tytso@....edu, adilger.kernel@...ger.ca, krisman@...labora.com, linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, kernel@...labora.com Subject: Re: [PATCH 1/2] fs: dcache: Handle case-exact lookup in d_alloc_parallel On 03/10/21 7:08 pm, Al Viro wrote: > On Wed, Sep 29, 2021 at 04:23:38PM +0530, Shreeya Patel wrote: >> There is a soft hang caused by a deadlock in d_alloc_parallel which >> waits up on lookups to finish for the dentries in the parent directory's >> hash_table. >> In case when d_add_ci is called from the fs layer's lookup functions, >> the dentry being looked up is already in the hash table (created before >> the fs lookup function gets called). We should not be processing the >> same dentry that is being looked up, hence, in case of case-insensitive >> filesystems we are making it a case-exact match to prevent this from >> happening. > NAK. What you are doing would lead to parallel calls of ->lookup() in the > same directory for names that would compare as equal. Which violates > all kinds of assumptions in the analysis of dentry tree locking. > > d_add_ci() is used to force the "exact" spelling of the name on lookup - > that's the whole point of that thing. What are you trying to achieve, > and what's the point of mixing that with non-trivial ->d_compare()? > Sending again as plain text... Hi Al Viro, This patch was added to resolve some of the issues faced in patch 02/02 of the series. Originally, the 'native', per-directory case-insensitive implementation merged in ext4/f2fs stores the case of the first lookup on the dcache, regardless of the disk exact file name case. This gets reflected in symlink returned by /proc/self/cwd. To solve this we are calling d_add_ci from the fs lookup function to store the disk exact name in the dcache even if an inexact-match string is used on the FIRST lookup. But this caused a soft hang since there was a deadlock in d_wait_lookup called from d_alloc_parallel. The reason for the hang is that d_same_name uses d_compare which does a case-insensitive match and is able to find the dentry name in the secondary hash table leading it to d_wait_lookup which would wait for the lookup to finish on that dentry causing a deadlock. To avoid the hang, we are doing a case-sensitive match using dentry_cmp here. Thanks > If it's "force to exact spelling on lookup, avoid calling ->lookup() on > aliases", d_add_ci() is simply not a good match.
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