Date: Tue, 17 Dec 2024 12:19:26 -0700 From: Alan Somers <asomers@freebsd.org> To: FreeBSD CURRENT <freebsd-current@freebsd.org> Subject: Why does namei return an exclusively locked vnode when LOCKSHARED is specified? Message-ID: <CAOtMX2hKxcE8ZWeEvQn58xGdD9H1wLsHoeQXfjEvkF4WACVYfA@mail.gmail.com>
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According to namei(9), namei should return a shared lock when
LOCKSHARED is specified. But in my experiments, it always seems to
return an exclusive lock instead. For example:
$ cd /usr/tests/sys/fs/fusefs
$ sudo dtrace -i 'fbt:fusefs:fuse_vnop_getattr:entry /pid==$target/
{printf("islocked=%#x", args[0]->a_vp->v_vnlock->lk_lock); stack();}'
-i 'fbt:kernel:vop_stdstat:entry /pid==$target/
{printf("islocked=%#x", args[0]->a_vp->v_vnlock->lk_lock);}' -c
"./getattr --gtest_filter=Getattr.attr_cache -v"
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from Getattr
[ RUN ] Getattr.attr_cache
INIT ino= 0
ACCESS ino= 1 mask=0x1
LOOKUP ino= 1 some_file.txt
GETATTR ino=42
[ OK ] Getattr.attr_cache (3 ms)
[----------] 1 test from Getattr (3 ms total)
[----------] Global test environment tear-down
[==========] 1 test from 1 test suite ran. (4 ms total)
[ PASSED ] 1 test.
CPU ID FUNCTION:NAME
3 19743 vop_stdstat:entry islocked=0x21
3 19743 vop_stdstat:entry islocked=0xfffff80004f16740
3 68298 fuse_vnop_getattr:entry islocked=0xfffff80004f16740
kernel`VOP_GETATTR_APV+0x90
kernel`vop_stdstat+0x147
kernel`VOP_STAT_APV+0x91
kernel`kern_statat+0x183
kernel`sys_fstatat+0x27
kernel`amd64_syscall+0x1af
kernel`0xffffffff8106cd5b
3 19743 vop_stdstat:entry islocked=0xfffff80004f16740
3 68298 fuse_vnop_getattr:entry islocked=0xfffff80004f16740
kernel`VOP_GETATTR_APV+0x90
kernel`vop_stdstat+0x147
kernel`VOP_STAT_APV+0x91
kernel`kern_statat+0x183
kernel`sys_fstatat+0x27
kernel`amd64_syscall+0x1af
kernel`0xffffffff8106cd5b
dtrace: pid 3554 has exited
>From that output, the first vop_stdstat is probably for the
mountpoint, and isn't what I'm concerned about. But the second two
are for a fusefs file. The LK_SHARE bit is not set, indicating an
exclusive lock. That's even though the call to namei in kern_statat
specifies LOCKSHARED | LOCKLEAF.
What am I missing?
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