{"id":346,"date":"2025-11-13T03:33:01","date_gmt":"2025-11-13T03:33:01","guid":{"rendered":"https:\/\/mitalgoswami.in\/?p=346"},"modified":"2025-11-13T03:33:27","modified_gmt":"2025-11-13T03:33:27","slug":"deadlock","status":"publish","type":"post","link":"https:\/\/mitalgoswami.in\/?p=346","title":{"rendered":"Deadlock"},"content":{"rendered":"\n

Definition:<\/strong><\/h2>\n\n\n\n

A deadlock<\/strong> is a situation in which two or more processes are waiting for resources<\/strong> that are held by each other<\/strong>, and none of them can proceed<\/strong>.
In short, it is a state of permanent waiting<\/strong>.<\/p>\n\n\n\n

\n\n\n\n

Example:<\/strong><\/h3>\n\n\n\n
    \n
  • Process P1<\/strong> holds Resource R1<\/strong> and waits for R2<\/strong>.<\/li>\n\n\n\n
  • Process P2<\/strong> holds Resource R2<\/strong> and waits for R1<\/strong>.<\/li>\n<\/ul>\n\n\n\n

    Both are waiting forever<\/strong> \u2014 this is a deadlock<\/strong>.<\/p>\n\n\n\n

    \n\n\n\n

    Four Necessary Conditions for Deadlock (Coffman Conditions):<\/strong><\/h3>\n\n\n\n
      \n
    1. Mutual Exclusion:<\/strong>
      Each resource can be used by only one process at a time.<\/li>\n\n\n\n
    2. Hold and Wait:<\/strong>
      A process is holding at least one resource and waiting for others.<\/li>\n\n\n\n
    3. No Preemption:<\/strong>
      A resource cannot be forcibly taken from a process.<\/li>\n\n\n\n
    4. Circular Wait:<\/strong>
      A circular chain of processes exists, each waiting for a resource held by the next process.<\/li>\n<\/ol>\n\n\n\n

      Deadlock occurs only if all four conditions<\/strong> hold simultaneously.<\/p>\n\n\n\n

      \n\n\n\n

      Deadlock Handling Methods<\/strong><\/h2>\n\n\n\n

      There are three main strategies<\/strong> to deal with deadlocks:<\/p>\n\n\n\n

      \n\n\n\n

      1. Deadlock Prevention<\/strong><\/h2>\n\n\n\n

      Definition:<\/strong><\/h3>\n\n\n\n

      Deadlock prevention means designing the system in such a way<\/strong> that at least one of the four necessary conditions cannot occur<\/strong>.<\/p>\n\n\n\n

      \n\n\n\n

      How it works:<\/strong><\/h3>\n\n\n\n

      We prevent<\/strong> deadlock by breaking<\/strong> one or more Coffman conditions:<\/p>\n\n\n\n

      Condition<\/th>Prevention Method<\/th><\/tr><\/thead>
      Mutual Exclusion<\/strong><\/td>Make resources sharable (where possible)<\/td><\/tr>
      Hold and Wait<\/strong><\/td>Process must request all resources at once<\/strong> before execution<\/td><\/tr>
      No Preemption<\/strong><\/td>Allow preemption \u2013 take resource from a process if needed<\/td><\/tr>
      Circular Wait<\/strong><\/td>Impose a strict ordering of resources<\/strong> (e.g., number them and request in increasing order)<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
      \n\n\n\n

      \u2705 Advantage:<\/strong><\/h3>\n\n\n\n
        \n
      • Prevents deadlock completely<\/strong><\/li>\n<\/ul>\n\n\n\n

        \u274c Disadvantage:<\/strong><\/h3>\n\n\n\n
          \n
        • May cause low resource utilization<\/strong> and process starvation<\/strong><\/li>\n<\/ul>\n\n\n\n
          \n\n\n\n

          2. Deadlock Avoidance<\/strong><\/h2>\n\n\n\n

          Definition:<\/strong><\/h3>\n\n\n\n

          Deadlock avoidance means the system dynamically examines resource allocation<\/strong> and decides whether to grant or wait<\/strong> for a request so that deadlock never occurs<\/strong>.<\/p>\n\n\n\n

          \n\n\n\n

          Technique:<\/strong><\/h3>\n\n\n\n

          The most common method is the Banker\u2019s Algorithm<\/strong> (used for multiple instances of resources).<\/p>\n\n\n\n

            \n
          • The system checks whether allocating a resource will keep it in a \u201csafe state\u201d<\/strong>.<\/li>\n\n\n\n
          • A safe state<\/strong> means there is at least one sequence<\/strong> of processes where each can finish execution.<\/li>\n<\/ul>\n\n\n\n

            If the system stays safe<\/strong>, it grants the resource;
            if not, the process must wait<\/strong>.<\/p>\n\n\n\n

            \n\n\n\n

            \u2705 Advantage:<\/strong><\/h3>\n\n\n\n
              \n
            • Ensures system will never enter a deadlock<\/strong>.<\/li>\n<\/ul>\n\n\n\n

              \u274c Disadvantage:<\/strong><\/h3>\n\n\n\n
                \n
              • Requires prior knowledge<\/strong> of maximum resource needs.<\/li>\n\n\n\n
              • Adds runtime overhead<\/strong> due to constant checking.<\/li>\n<\/ul>\n\n\n\n
                \n\n\n\n

                3. Deadlock Detection and Recovery<\/strong><\/h2>\n\n\n\n

                Definition:<\/strong><\/h3>\n\n\n\n

                In this method, the system does not prevent or avoid<\/strong> deadlocks.
                Instead, it allows<\/strong> them to occur and then detects<\/strong> and recovers<\/strong> from them.<\/p>\n\n\n\n

                \n\n\n\n

                How it works:<\/strong><\/h3>\n\n\n\n
                  \n
                1. Detection:<\/strong>\n
                    \n
                  • The system uses algorithms to periodically check<\/strong> for circular waits among processes.<\/li>\n\n\n\n
                  • A wait-for graph<\/strong> is often used (especially in single-instance systems).<\/li>\n\n\n\n
                  • If a cycle<\/strong> exists in the graph \u2192 deadlock detected<\/strong>.<\/li>\n<\/ul>\n<\/li>\n\n\n\n
                  • Recovery:<\/strong>\n
                      \n
                    • Once detected, the system recovers<\/strong> using one of these methods:\n
                        \n
                      • Terminate processes<\/strong> (one or more) to break the cycle.<\/li>\n\n\n\n
                      • Preempt resources<\/strong> and give them to other processes.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<\/li>\n<\/ol>\n\n\n\n
                        \n\n\n\n

                        \u2705 Advantage:<\/strong><\/h3>\n\n\n\n
                          \n
                        • Efficient use<\/strong> of resources (no restrictions upfront)<\/li>\n<\/ul>\n\n\n\n

                          \u274c Disadvantage:<\/strong><\/h3>\n\n\n\n
                            \n
                          • Overhead<\/strong> in detection and recovery<\/li>\n\n\n\n
                          • May cause data loss<\/strong> or process restart<\/strong><\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"

                            Definition: A deadlock is a situation in which two or more processes are waiting for resources that are held by each other, and none of them can proceed.In short, it is a state of permanent waiting. Example: Both are waiting forever \u2014 this is a deadlock. Four Necessary Conditions for Deadlock (Coffman Conditions): Deadlock occurs […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[5],"tags":[],"class_list":["post-346","post","type-post","status-publish","format-standard","hentry","category-unix"],"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/mitalgoswami.in\/index.php?rest_route=\/wp\/v2\/posts\/346","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/mitalgoswami.in\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/mitalgoswami.in\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/mitalgoswami.in\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/mitalgoswami.in\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=346"}],"version-history":[{"count":1,"href":"https:\/\/mitalgoswami.in\/index.php?rest_route=\/wp\/v2\/posts\/346\/revisions"}],"predecessor-version":[{"id":347,"href":"https:\/\/mitalgoswami.in\/index.php?rest_route=\/wp\/v2\/posts\/346\/revisions\/347"}],"wp:attachment":[{"href":"https:\/\/mitalgoswami.in\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=346"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/mitalgoswami.in\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=346"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/mitalgoswami.in\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=346"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}