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originally observed with psmisc-22.20-15.el7.s390x
+++ This bug was initially created as a clone of Bug #1509243 +++
Description of problem:
I have found a problem during mariadb startup, which turned out to be caused by a `fuser` call that timed out due to broken NFS.
While that's a string of unfortunate conditions, making `fuser` behave differently would be IMHO a good thing.
The problem is when a file doesn't exist, instead of bailing out immediately, fuser tries to stat tons of other things, and stating a stale NFS mount leads to hanging on it.
fuser shouldn't do this - I don't know what sense does it make during normal operation (if the file exists), it still would be nice if it could be skipped and so fuser doesn't get blocked by NFS.
But I'm pretty sure it doesn't make any sense when the file doesn't exist ... why to wait for "Specified filename ... does not exist." message until the system is scanned while it can be spit out immediately?
Version-Release number of selected component (if applicable):
psmisc-23.1-2.fc27.x86_64
How reproducible:
always
Steps to Reproduce:
0. have some nfs mounts
[supposing 'kravina' doesn't exist]
1. strace fuser kravina 2>&1 | grep your/mount
Actual results:
stat("your/mount", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
Expected results:
empty output, grep doesn't match your/mount
Additional info:
Hello Karel,
so if I understand it correctly, the problem is not that fuser hangs, but it is slowed down by {non,slow-}responding mounts on the system, which is unnecessary from your point of view and it should first verify the existence of a file, right?
(In reply to Jan Rybar from comment #2)
> Hello Karel,
>
> so if I understand it correctly, the problem is not that fuser hangs, but it
> is slowed down by {non,slow-}responding mounts on the system, which is
> unnecessary from your point of view and it should first verify the existence
> of a file, right?
Hi, I'm not sure if I recall correctly but I believe what we observed was that it hanged indefinitely when NFS was unresponsive
I don't know how does it work internally, but I'd think that it shouldn't try to touch the mountpoint at all, no matter if the file exists or not ... and if that is unavoidable, then yes, it should try that only after checking that the file exists
Created attachment 1536393[details]
Proposed patch
Due to missing NULLptr sanity checks in scanning functions, fuser runs excessive number of stats on entire /proc several times with no potential of finding anything.
There is a case that this activity causes NFS to stall and get stuck even when finding for a non-existing file.
Patch sent upstream.
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.
https://access.redhat.com/errata/RHBA-2019:2225