Fair spinlock with protection against thread preemption

A system comprising:

• one or more processors;
• and
• a memory comprising code stored thereon that, when executed, performs a method comprising:
• a first thread TH1 acquiring a spinlock;
• while TH1 holds the spinlock, a second thread TH2 and a third thread TH3 each attempting to acquire the spinlock;
• responsive to TH2 and TH3 attempting to acquire the spinlock while held by TH1, adding TH2 and TH3 to a queue of threads waiting to acquire the spinlock, wherein TH2 and TH3 are added to the queue based on a FIFO (first in first out) ordering in which threads attempt to acquire the spinlock, where TH1 has a first position or index P1 in the FIFO ordering, TH2 has a second position or index P2 in the FIFO ordering consecutively following TH1, and TH3 has a third position or index P3 in the FIFO ordering consecutively following TH2;
• while TH1 holds the spinlock, pre-empting TH2 so that TH2 becomes inactive;
• while TH2 is inactive and waiting at the second position or index P2 in the FIFO ordering of the queue, TH1 releasing the spinlock so that the spinlock is free;
• detecting that the spinlock is free and that TH3 is waiting in the queue at the third position or index P3 immediately following the second position or index P2 of TH2, where TH2 is next, in the FIFO ordering of threads in the queue, to acquire the spinlock after released by TH1;
• responsive to said detecting, determining that TH2 is inactive and has been pre-empted, and performing first processing including:
• saving a copy of the second position or index P2 in saved information;
• cancelling the second position or index P2 of TH2 in the FIFO ordering;
• and
• TH3 acquiring the spinlock;
• subsequent to said canceling, rescheduling TH2 for execution;
• and
• responsive to said rescheduling, TH2 resuming execution, wherein said TH2 resuming execution includes TH2 performing second processing including:
• determining, using the copy of the second position or index P2, that the second position or index P2 of TH2 in the FIFO ordering is canceled;
• and
• responsive to determining that the second position or index P2 of TH2 in the FIFO ordering is canceled, assigning TH2 a fourth position or index P4 in the FIFO ordering, wherein P4 has a corresponding position or index in the FIFO ordering subsequent to the third position or index P3.

A non-transitory computer-readable media comprising code stored thereon that, when executed, performs a method comprising:

• a first thread TH1 acquiring a spinlock;
• while TH1 holds the spinlock, a second thread TH2 and a third thread TH3 each attempting to acquire the spinlock;
• responsive to TH2 and TH3 attempting to acquire the spinlock while held by TH1, adding TH2 and TH3 to a queue of threads waiting to acquire the spinlock, wherein TH2 and TH3 are added to the queue based on a FIFO (first in first out) ordering in which threads attempt to acquire the spinlock, where TH1 has a first position or index P1 in the FIFO ordering, TH2 has a second position or index P2 in the FIFO ordering consecutively following TH1, and TH3 has a third position or index P3 in the FIFO ordering consecutively following TH2;
• while TH1 holds the spinlock, pre-empting TH2 so that TH2 becomes inactive;
• while TH2 is inactive and waiting at the second position or index P2 in the FIFO ordering of the queue, TH1 releasing the spinlock so that the spinlock is free;
• detecting that the spinlock is free and that TH3 is waiting in the queue at the third position or index P3 immediately following the second position or index P2 of TH2, where TH2 is next, in the FIFO ordering of threads in the queue, to acquire the spinlock after released by TH1;
• responsive to said detecting, determining that TH2 is inactive and has been pre-empted, and performing first processing including:
• saving a copy of the second position or index P2 in saved information;
• cancelling the second position or index P2 of TH2 in the FIFO ordering;
• and
• TH3 acquiring the spinlock;
• subsequent to said canceling, rescheduling TH2 for execution;
• and
• responsive to said rescheduling, TH2 resuming execution, wherein said TH2 resuming execution includes TH2 performing second processing including:
• determining, using the copy of the second position or index P2, that the second position or index P2 of TH2 in the FIFO ordering is canceled;
• and
• responsive to determining that the second position or index P2 of TH2 in the FIFO ordering is canceled, assigning TH2 a fourth position or index P4 in the FIFO ordering, wherein P4 has a corresponding position or index in the FIFO ordering subsequent to the third position or index P3.