On Sun, Dec 13, 2020 at 08:30:40AM -0600, Eric W. Biederman wrote: > Stephen Brennan <stephen.s.brennan@xxxxxxxxxx> writes: > > > The pid_revalidate() function requires dropping from RCU into REF lookup > > mode. When many threads are resolving paths within /proc in parallel, > > this can result in heavy spinlock contention as each thread tries to > > grab a reference to the /proc dentry lock (and drop it shortly > > thereafter). > > I am feeling dense at the moment. Which lock specifically are you > referring to? The only locks I can thinking of are sleeping locks, > not spinlocks. Stephen may have a better answer than this, but our mutex implementation spins if the owner is still running, so he may have misspoken slightly. He's testing on a giant system with hundreds of CPUs, so a mutex is going to behave like a spinlock for him. > Why do we need to test flags here at all? > Why can't the code simply take an rcu_read_lock unconditionally and just > pass flags into do_pid_update_inode? Hah! I was thinking about that possibility this morning, and I was going to ask you that question.
