Q: nr_threads locking
Subsystems (3)
Hi Andrew,
According to the comments, nr_threads is protected by lock_kernel, but do_fork() runs without the bkl for ages.
Would it be possible to use your percpu_counters for nr_threads? It seems to be used only to guard against fork bombs and for i_nlink of /proc.
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Manfred
Quoted 8 lines · Manfred Spraul
Hi Andrew,
According to the comments,
nr_threadsis protected bylock_kernel, butdo_fork()runs without the bkl for ages.
Would it be possible to use yourpercpu_countersfornr_threads? It seems to be used only to guard against fork bombs and fori_nlinkof /proc.
It would be possible, yes.
But thread creation is a "rare" event compared to pagefaults and syscalls. An atomic_t will be OK there.
Quoted 5 lines · Andrew Morton
It would be possible, yes.
But thread creation is a "rare" event compared to pagefaults and syscalls. An
atomic_twill be OK there.
Actually, the code is correct. The documentation it bogus. lock_kernel() never achieved any protection: the copy_xy() functions can sleep.
What about the attached docu update?
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Manfred
kernel/fork.c +6 -4
@@ -43,7 +43,9 @@extern int copy_semundo (unsigned long clone_flags,struct task_struct *tsk);extern void exit_semundo (struct task_struct *tsk); -/* The idle threads do not count.. */ +/* The idle threads do not count.. +* Protected by write_lock_irq(&tasklist_lock) +*/ int nr_threads;int max_threads;
@@ -792,9 +794,9 @@atomic_inc (&p->user->processes);/* -* Counter increases are protected by -* the kernel lock so nr_threads can't -* increase under us (but it may decrease). +* If multiple threads are within copy_process(), then this check +* triggers too late. This doesn't hurt, the check is only there +* to stop root fork bombs. */ if (nr_threads >= max_threads) goto bad_fork_cleanup_count;