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POrace condition between wait_event and wake_up
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copied!<p>I have a driver that wants to send notification to the user about a status change. In the current implementation it uses the proc filesystem to do so. The read process loops around a <code>read()</code> to the proc filesystem. The <code>read()</code> blocks with <code>wait_event_interruptible()</code> until the kernel gets an interrupt which causes the <code>write_new_data()</code> function to <code>call wake_up_interruptible()</code>. Here's the basic code (removed all unneeded clutter):</p> <pre><code>static int flag=0; DECLARE_WAIT_QUEUE_HEAD(info_wq); //user process call read() on /proc/myfile to get to this function int my_proc_read (struct file *filp, char *buf, size_t count, loff_t *pos) { wait_event_interruptible(info_wq, flag != 0); flag = 0; //copy buffers to user return 0; } //when an interrupt comes it schedules this function on the systems' work queue void write_new_data () { //fill buffer with data flag = 1; wake_up_interruptible(&info_wq); } </code></pre> <p>Now consider the following flow:</p> <ol> <li>User process calls <code>read()</code>, then waits.</li> <li>interrupt occurs -> <code>write_new_data()</code> is called. writes data and calls <code>wake_up_interruptible()</code>.</li> <li><code>read()</code> is awaken, reads data but process has not rerun read (wasn't scheduled to run, didn't get to it because of the next interrupt...).</li> <li>interrupt occurs -> <code>write_new_data()</code> is triggered again, calls <code>wake_up_interruptible()</code> but no waiting thread is waiting...</li> <li>process calls read and blocks.</li> </ol> <p>Note: this all happens on a uni-processor system. Also there is only one thread reading and one thread writing new data.</p> <p>How can I avoid missing the second interrupt? (One solution is to use netlink sockets but I was wondering if there is a way to do it in /proc land)</p>

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