Remaining system calls to make MPSAFE -- NTP, VFS, AIO, linux/etc

Robert Watson rwatson at FreeBSD.org
Sat May 28 07:01:49 PDT 2005


Jeff, Maxime:

I spent a bit of time today walking syscalls.master to try and clean up 
straggling non-MPSAFE system calls.  I was able to add the MPSAFE flag in 
several cases:

fhopen(), fhstat(), fhstatfs(), freebsd4_fhstatfs(), quotactl()

   Add unconditional Giant acquisition/drop to vfs_syscalls.c system call
   code.  This needs refinement to match the conditional Giant use present
   in other VFS system calls.

freebsd4_getfsstat(), olseek(), ostat(), olstat(), otruncate(), 
oftruncate(), freebsd4_statfs(), freebsd4_fstatfs()

   The kern_ and non-compat system calls implementing these compatibility
   system calls was already MPSAFE, so I could simply add the 'M' flag to
   syscalls.master.

After doing this, the following FreeBSD system calls are still not marked 
MPSAFE:

extattr_*()

   All extended attribute system calls are not marked as MPSAFE.  A first
   step here would be to push Giant into their system call paths, but
   something a bit more refined would be desirable and should be possible.

nfsclnt()

   I'll take a look through this today and see what I can do. I suspect the
   dependencies here are easy to clean up.

ntp_gettime()

   Other NTP-related system calls appear safe.  I've pinged Poul-Henning
   and Mark to see if we can get this fixed; I imagine that is fairly
   straight forward, but don't know.

ogetdirentries()

   This compat call doesn't reuse the current system call code, so needs to
   be locked down.  getdirentries() is probably a reasonable model for a
   starting point, but this requires a bit of work.

mount(), unmount(), nmount()

   All of the mount-related calls aren't marked MPSAFE.  I'm not very
   informed on this front, but other than possible entanglement in the boot
   and shutdown paths, things may actually be not too bad here.

aio_*(), lio_*()

   AIO is not MPSAFE, and qute a bit of work is required here.

There's probably quite a bit of low-hanging fruit in the compatibility ABI 
system call tables.  In particular, quite a few calls can probably be 
marked MPSAFE on the basis that they call MPSAFE code and do little or no 
work.  In many cases we are probably unnecessarily acquiring or recursing 
Giant as things stand.  In other places, more work will be required, where 
the compat code actually implements substantial services or calls, such as 
additional file system system calls not present in the FreeBSD interface.

The motivation behind this revisiting of syscalls.master is that I'm about 
to change the file format to add the audit event record identifier field 
for each system call, and I had hoped to simplify makesyscalls.sh before 
doing so.  I think we're not quite close enough to there yet for me to do 
that before merging the Audit code, but I think we're actually quite close 
to getting rid of Giant acquisition in the general system call handler, 
which would be really great.

Robert N M Watson


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