Messages in this thread | | | Date | Fri, 9 Oct 2009 13:55:43 +0800 | Subject | select system call's implementation may have some bug, need your help and confirm !!! | From | wu Jianfeng <> |
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A process may sleep for ever when he call select system call. In detail, if the process was scheduled out just at the point it set its state to TASK_INTERRUPTIBLE.
The events that cause the process to be scheduled out is(in preempt kernel) : 1) time interrupt and the process's time slice is exhausted. 2) an interrupt accured, and wake up another process with high priority.
see the comments after "##"
int do_select(int n, fd_set_bits *fds, s64 *timeout) { struct poll_wqueues table; poll_table *wait; int retval, i;
rcu_read_lock(); retval = max_select_fd(n, fds); rcu_read_unlock();
if (retval < 0) return retval; n = retval;
poll_initwait(&table); wait = &table.pt; if (!*timeout) wait = NULL; retval = 0; for (;;) { unsigned long *rinp, *routp, *rexp, *inp, *outp, *exp; long __timeout;
set_current_state(TASK_INTERRUPTIBLE); ## here set the process state TASK_INTERRUPTIBLE
## if the process was scheduled out here, then the process will never can be waked up, because it has not been attached to any file 's wait queue.
inp = fds->in; outp = fds->out; exp = fds->ex; rinp = fds->res_in; routp = fds->res_out; rexp = fds->res_ex;
for (i = 0; i < n; ++rinp, ++routp, ++rexp) { unsigned long in, out, ex, all_bits, bit = 1, mask, j; unsigned long res_in = 0, res_out = 0, res_ex = 0; struct file_operations *f_op = NULL; struct file *file = NULL;
in = *inp++; out = *outp++; ex = *exp++; all_bits = in | out | ex; if (all_bits == 0) { i += __NFDBITS; continue; }
for (j = 0; j < __NFDBITS; ++j, ++i, bit <<= 1) { if (i >= n) break; if (!(bit & all_bits)) continue; file = fget(i); if (file) { f_op = file->f_op; mask = DEFAULT_POLLMASK; if (f_op && f_op->poll) mask = (*f_op->poll)(file, retval ? NULL : wait); fput(file); if ((mask & POLLIN_SET) && (in & bit)) { res_in |= bit; retval++; } if ((mask & POLLOUT_SET) && (out & bit)) { res_out |= bit; retval++; } if ((mask & POLLEX_SET) && (ex & bit)) { res_ex |= bit; retval++; } } cond_resched(); } if (res_in) *rinp = res_in; if (res_out) *routp = res_out; if (res_ex) *rexp = res_ex; } wait = NULL; if (retval || !*timeout || signal_pending(current)) break; if(table.error) { retval = table.error; break; }
if (*timeout < 0) { /* Wait indefinitely */ __timeout = MAX_SCHEDULE_TIMEOUT; } else if (unlikely(*timeout >= (s64)MAX_SCHEDULE_TIMEOUT - 1)) { /* Wait for longer than MAX_SCHEDULE_TIMEOUT. Do it in a loop */ __timeout = MAX_SCHEDULE_TIMEOUT - 1; *timeout -= __timeout; } else { __timeout = *timeout; *timeout = 0; } __timeout = schedule_timeout(__timeout); if (*timeout >= 0) *timeout += __timeout; } __set_current_state(TASK_RUNNING);
poll_freewait(&table);
return retval; }
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