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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><div style='border:none;border-left:solid blue 1.5pt;padding:0in 0in 0in 4.0pt'><div><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> kernelnewbies-bounces@kernelnewbies.org [mailto:kernelnewbies-bounces@kernelnewbies.org] <b>On Behalf Of </b>Prajosh Premdas<br><b>Sent:</b> Wednesday, November 02, 2011 2:02 AM<br><b>To:</b> kernelnewbies@kernelnewbies.org<br><b>Subject:</b> spin_lock behavior<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Hi <o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>I wrote a sample module which looked like this and i expected the module to hang as i have used 2 spin_lock simultaneously. But i find the print after the second spin_lock being printed and the module works perfectly fine. Can any body please help me in understanding this? I got this problem from a driver i wrote and had a typo "spin_lock" instead of "spin_unlock" the driver worked fine and i caught the mistake only during a review <o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><div><p class=MsoNormal><i>#include <linux/module.h></i><o:p></o:p></p></div><div><p class=MsoNormal><i>#include <linux/spinlock.h></i><o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><i>struct sp_q {</i><o:p></o:p></p></div><div><p class=MsoNormal><i>spinlock_t spinlock;</i><o:p></o:p></p></div><div><p class=MsoNormal><i>uint8_t data;</i><o:p></o:p></p></div><div><p class=MsoNormal><i>};</i><o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><i>static int __init sp_lck_init(void)</i><o:p></o:p></p></div><div><p class=MsoNormal><i>{</i><o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><i> struct sp_q test;</i><o:p></o:p></p></div><div><p class=MsoNormal><i> </i><o:p></o:p></p></div><div><p class=MsoNormal><i> /* Spin lock Init */</i><o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><i> spin_lock_init(&test.spinlock);</i><o:p></o:p></p></div><div><p class=MsoNormal><i> </i><o:p></o:p></p></div><div><p class=MsoNormal><i> printk("<1>Test start\n");</i><o:p></o:p></p></div><div><p class=MsoNormal><i> </i><o:p></o:p></p></div><div><p class=MsoNormal><i> spin_lock(&test.spinlock);</i><o:p></o:p></p></div><div><p class=MsoNormal><i> test.data = 0; </i><o:p></o:p></p></div><div><p class=MsoNormal><i> spin_lock(&test.spinlock); </i><o:p></o:p></p></div><div><p class=MsoNormal><i> </i><o:p></o:p></p></div><div><p class=MsoNormal><i> printk("<1>How come???\n");</i><o:p></o:p></p></div><div><p class=MsoNormal><i> return 0;</i><o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><i>}</i><o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><i>static void __exit sp_lck_exit(void)</i><o:p></o:p></p></div><div><p class=MsoNormal><i>{</i><o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><i>}</i><o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><i>module_init(sp_lck_init);</i><o:p></o:p></p></div><div><p class=MsoNormal><i>module_exit(sp_lck_exit);</i><o:p></o:p></p></div><div><p class=MsoNormal><i>MODULE_LICENSE("GPL");</i><o:p></o:p></p></div></div><div><div><p class=MsoNormal><o:p> </o:p></p></div><p class=MsoNormal>-- <br>Regards,<br><br>Prajosh Premdas<o:p></o:p></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>On single processor systems, spin_lock() doesn’t actually spin on anything.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>The term spin_lock() is a bit of a misnomer in my opinion as it implies that some sort of spinning will go on if the lock has already been taken.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>It really means, sort of, “disable preemption, then serialize access on this variable among other threads on other CPUs, and I promise not to try to take this lock in any top or bottom half context on this CPU.”<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Of course, that would make for an unwieldy function name, so its call spin_lock().<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Jeff Haran<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p></div></div></div><font face="monospace"></font></body></html>